Bull. Mater. Sci., Vol. 26, No. 7, December 2003, pp. 741–744. © Indian Academy of Sciences.

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Comparative study of dielectric properties of MgNb2O6 prepared by molten salt and ceramic method

VISHNU SHANKER and ASHOK K GANGULI* Department of Chemistry, Indian Institute of Technology, New Delhi 110 016, India MS received 10 July 2003 Abstract. Magnesium niobate (MgNb2O6) powder was synthesized by the conventional ceramic route as well as by the molten salt route using a eutectic mixture of NaCl–KCl as the salt and Mg(NO3)2⋅⋅6H2O and TiO2 as the starting materials. Pure phase of MgNb2O6 could be obtained by the molten salt method at 1100°C. How-ever, in ceramic method the pure phase of MgNb2O6 was obtained by heating at 1025°C for 20 h. On sintering at 1100°C the dielectric constant and dielectric loss of MgNb2O6 obtained by the molten salt method was found to be 19⋅⋅5 and 0⋅⋅004 at 100 kHz at room temperature. Lower values were obtained for these oxides pre-pared by the ceramic route, 16⋅⋅6 and 0⋅⋅000518, respectively. In both cases the dielectric constant was quite stable with frequency. Keywords. Ceramics; molten salt route; dielectric constant; dielectric loss.

1. Introduction

In recent years there has been considerable interest in developing dielectric ceramics for their use as resonators at microwave frequency due to the very fast growth of mobile communication systems (Ouchi and Kawashima 1985; Wakino et al 1990). Researchers have focussed attention towards the development of materials with high dielectric constant which would allow the reduction of the size of resonators. Since the wavelength (λ) of a di-electric resonator is inversely proportional to √εr, where εr is the relative dielectric constant of the resonator (Lee et al 1997a; Hsu et al 2003), some typical examples of such materials are found in the BaO–Re2O3–TiO2 (Sato et al 1987) and (Pb1–xCax)ZrO3 (Wakino et al 1984) sys-tems. Many complex perovskite compounds such as Ba (Mg1/3Ta2/3)O3 (Nomura et al 1982) and Ba(Zn1/3Ta2/3)O3 (Kawashima et al 1983) are also known to have good microwave dielectric properties. Oxides related to the AB2O6 structure, where A is an alkaline-earth metal and B is either Nb or Ta, have also been studied for their microwave dielectric properties (Lee et al 1997a, b; Maeda et al 1987). Most of the nio-bium oxides related to AB2O6 have the columbite struc-ture with Pbcn space group while the tantalates have a variety of related structures, which depends on the A- site cation. MgNb2O6 has been earlier investigated by Maeda et al (1987) and Lee et al (1997a). The microwave dielectric properties of this compound (εr ~ 21⋅4, Q × f ~ 93,800 GHz and τf ~ – 70 ppm/°C at 1300°C; Lee et al

1997a) are suitable for applications in dielectric resona-tors. Lee et al (1997a) have earlier synthesized MgNb2O6 at temperatures from 900 to 1025°C by high purity raw materials of Nb2O5 and MgO, by the ceramic route and sintered at 1300°C for 2 h in air. However, it has been reported that MgNb2O6 was synthesized at 1100°C by using Mg(NO3)2⋅6H2O as the source of Mg through con-ventional ceramic method (Thirumal and Ganguli 2001). Due to the increasing demand of these materials there has been considerable research on development of easy and cost-effective synthetic methods for dielectric mate-rials. The molten-salt synthesis or flux route had been used earlier to synthesize various oxides like BaFe12O19 (Arendt 1973), Pb(Mg1/3Nb2/3)O3 (Yoon et al 1995), Ba (Mg1/3Nb2/3)O3 and Ba(Zn1/3Nb2/3)O3 (Thirumal et al 2001). The flux route allows melt–solid reactions which are faster due to small diffusion distances and higher mobi-lity of oxides in the melt. So reactions may be performed at lower temperatures. In this paper, we report the synthesis of MgNb2O6 using the flux method. We have also synthesized the above oxide at lower heating conditions using the ceramic route by avoiding MgO or MgCO3 (which have been used earlier) by using hexahydrated magnesium nitrate (Mg(NO3)2⋅6H2O) instead. A comparative study of the dielectric properties of MgNb2O6 obtained by the ceramic route and by the molten salt route is discussed.

2. Experimental

Stoichiometric ratio of Mg(NO3)2⋅6H2O (Merck 99%), and TiO2 (Fluka 99%), NaCl (Merck 99%) and KCl

*Author for correspondence

Vishnu Shanker and Ashok K Ganguli

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(Merck 99%) were used for the synthesis of MgNb2O6. In the molten salt method stoichiometric amount of Mg(NO3)2⋅6H2O (Merck 99%), and TiO2 (Fluka 99%) were used along with eutectic mixture (1 : 1) of NaCl–KCl. The weight of reactant to the salt mixture was in the ratio of 2 : 1. The reactants were thoroughly ground in an agate mortar and then loaded in alumina crucibles in a programmable furnace at 900°C for 6 h. The product was then washed several times with chloride-free hot distilled water to remove salt. The powder was further heated at 1000°C for 20 h and 1100°C for 24 h with intermittent grinding. In the ceramic method, stoichiometric ratio of Mg(NO3)2⋅6H2O, and TiO2 were taken in an agate mortar and properly homogenized. The homogenized mixture was first heated at 900°C for 12 h and was further heated at 1025°C for 20 h. The powder samples of MgNb2O6 synthesized by molten salt method and ceramic method were treated with polyvinyl alcohol (PVA) and then pelle-tized at a pressure of 8 tonnes and sintered at 1100°C for 12 h. Powder X-ray diffraction studies were carried out after each stage of heating on a Bruker D8 advance X-ray diffractometer. The refined lattice parameters were obtained by a least square fit to the observed d-values. Scanning electron micrographs (SEM) were obtained on the sintered disks using a Cambridge Stereoscan 360 electron microscope. The dielectric properties (dielectric constant and dielectric loss) were measured on sintered pellets (coated with Ag-paste as electrodes) using an HP 4284L LCR meter in the frequency range of 50–500 kHz,

at varying temperatures (35–200°C). The density of sin-tered pellets of MgNb2O6 were measured by Archimedes principle and the relative density was found to be around 91% and 92%, respectively of theoretical density.

3. Results and discussion

Magnesium niobate was synthesized by both the ceramic and molten salt methods. These compounds were ana-lysed by powder X-ray diffraction. In case of the molten salt synthesis, after the first heating stage (900°C for 6 h) nearly 60% of Mg4Nb2O9 was present which decreases to 12% after heating at 1100°C. Pure phase of MgNb2O6

was obtained after a further heating at 1100°C for 12 h (figure 1). Magnesium niobate synthesized by the cera-mic route led to a biphasic mixture with 12% impurity of Mg4Nb2O9 along with the major phase after heating at 900°C. Further heating at 1025°C led to a single phase. The powder diffraction pattern of MgNb2O6 prepared by ceramic method at 1025°C has been shown in figure 2. All the peaks could be indexed satisfactorily in the ortho-rhombic cell as known for the columbite structure of MgNb2O6. The refined lattice parameters were found to be as follows: ‘a’ = 14⋅170(6), ‘b’ = 5⋅680(3), ‘c’ = 5⋅022(2). This is similar to a previous study (Thirumal and Ganguli 2001) where single phase formation occurred at a lower temperature (1100°C) using Mg(NO3)2⋅6H2O as a starting material. MgNb2O6 is normally prepared at 1300°C using MgO and Nb2O5 oxides.

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Figure 1. Powder X-ray diffraction pattern of MgNb2O6 prepared by the molten salt method after 1100°C heating.

Dielectric properties of magnesium niobate

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Scanning electron micrographs (SEM) of MgNb2O6 prepared by the molten salt method at 1100°C show that the grain boundaries are not well defined and the grain size was found to be around 3–5 µm (figure 3). However, in the oxide obtained by the ceramic route the grain size was around 1⋅5 to 2⋅5 µm (figure 4). Earlier reports have shown that the grain size of MgNb2O6 prepared by the same (ceramic) method was around 7–10 µm when sin-tered at 1200°C (Thirumal and Ganguli 2001). There is considerable increase in the grain size on increasing the sintering temperature from 1100–1200°C.

Detailed studies on the dielectric properties of MgNb2O6 was carried out on pellets sintered at 1100°C in the tem-perature range 35–200°C and frequency range, 50 Hz–500 kHz. The variation of the dielectric properties with frequency at room temperature for MgNb2O6 obtained by the two methods are shown in figures 5 and 6. The di-electric constant (ε) was found to be 19⋅5 and dielectric loss (D) was 0⋅004 at 100 kHz for MgNb2O6 synthesized by the molten salt route. However, in the ceramic method the dielectric constant and dielectric loss were both found to be lower (ε = 16⋅6; D = 0⋅0005) at the same frequency.

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Figure 2. Powder X-ray diffraction pattern of MgNb2O6 prepared by ceramic method at 1025°C.

Figure 3. Scanning electron micrograph of MgNb2O6 sintered at 1100°C (molten salt method).

Figure 4. Scanning electron micrograph of MgNb2O6 sintered at 1100°C (ceramic method).

Vishnu Shanker and Ashok K Ganguli

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Earlier report on MgNb2O6 prepared by the ceramic route and sintered at 1200 °C showed dielectric constant and di-electric loss of 24 and 0⋅005, respectively (Thirumal and Ganguli 2001). This increase in the dielectric constant may be due to the difference in the grain size obtained at two sintering temperatures, 1⋅5–2⋅5 µm at 1100°C and 7–10 µm at 1200°C. The dielectric constant (ε) was found to be stable in the range of 10 kHz–500 kHz irrespective of the preparative method in the present study. The di-electric loss (D) shows very stable value in the frequency

range 1–200 kHz. However, there is an increase in the dielectric loss beyond 300 kHz till 500 kHz, which may suggest a well-defined loss peak at higher frequencies. It may be noted that sintering at higher temperatures (1200°C) does not show such an increase in the loss at higher frequencies (Thirumal and Ganguli 2001).

4. Conclusions

We could obtain pure phase of MgNb2O6 using Mg (NO3)⋅6H2O by both the molten salt method and ceramic method. The dielectric constant and loss were found to be slightly higher in the oxide obtained by the molten salt method as compared to ceramic method sintered at the same temperature (1100°C). This may be associated with the larger grain size in the oxide prepared by the molten salt method.

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Figure 5. Variation of the dielectric constant (ε) and dielectric loss (D) with frequency at room temperature for MgNb2O6 prepared by the molten salt method.

Figure 6. Variation of the dielectric constant (ε) and dielectric loss (D) with frequency at room temperature for MgNb2O6 prepared by the ceramic method.